See there is no restriction for you. You can give the exam. However it all depends upon the M.Sc you want to pursue. i dont think M.Sc in Computer Science is there through JAM. Yes M.Sc Mathematics is available for which you need to have Maths, Physcis as compusory subjects in graduation and CS as an optional subject. Check the eligibility here to know which M.Sc and what requirements are allowed - http://jam.iitb.ac.in/meq1.html

I see IIT Guwahati, Hyderabad and Dhambad offering M.Sc in Mathematics & Computing. Pls check that out.